JP2009-236190A discloses a hybrid construction machine in which a hydraulic motor is rotated utilizing return oil from a boom cylinder and a generator is rotated by a rotational force of the hydraulic motor. In this construction, a regeneration flow control valve is provided at a passage connecting a piston-side chamber of the boom cylinder and an operation valve and the regeneration flow control valve is connected to the hydraulic motor.
Further, a lowering speed of the boom cylinder is controlled while the regeneration flow is controlled by the regeneration flow control valve, and the flow of the return oil from the boom cylinder other than the regeneration flow is partly recycled to a rod-side chamber of the boom cylinder and returned to a tank via the operation valve.